Method and apparatus for web tension control



Jan. 30, 1968 A. s. BAHRANI 3,365,298

METHOD AND APPARATUS FOR WEB TENSION CONTROL x Filed Nov. 22,-1965 INVENTOR Abdul S. Buhroni BY ATTORNEYS United States Patent Ofiice 3,366,298 METHUD AND APPARATUS FOR WEB TENSION CONTROL Abdul S. Bahrani, Colerain Township, Hamilton County, Ohio, assignor to The Procter & Gamble Company, Cincinnati, Ohio, a corporation of Ohio Filed Nov. 22, 1965, Ser. No. 508,922 8 Claims. (Cl. 226-1) ABSTRACT OF THE DISCLOSURE A method and apparatus for controlling the tension of moving webs by passing the web over one or more elongated, air lubricated turning bars in which an enclosure is positioned next to and on the same side of the web as the turning bar. The enclosure has a longitudinal slot which is positioned adjacent the web as the latter approaches the bar and is oriented to direct a stream of air tangentially with respect to the bar and toward the web to provide a friction-reducing air film between the web and the turning bar. The tension in the web is controlled by varying the rate of air flow through the slot.

This invention relates to a method and apparatus for handling webs and more particularly to a method and apparatus for controlling the tension in a web of porous lightweight tissue or the like as it moves along a predetermined path of travel as an incident of converting operations.

Tension control devices of the type used heretofore have included spring-biased bars which bear against an unsupported portion of a moving web to maintain the web in a relatively taut condition and mechanical or electrical brakes acting on the unwinding roll to place the unwinding web in tension. These devices generally require skilled operators to observe the operaton and to adjust the equipment as necessary to provide the tension desired. It is essential that such webs be under some tension so that they can be easily and positively handled. In the absence of tension, the webs would be slack and could not be controlled or made to accurately follow the guiding apparatus provided to convey them through the various operations they are to undergo. As a matter of fact, webs which are not under tension also may wrinkle or fold over themselves in some undesirable fashion.

In addition to the tension imposed by tension control devices, the passage of the web around turning or guide bars, such as are frequently used to change the direction of the web or shift it laterally, results in a frictional drag force which further adds to the web tension. This force is particularly significant when lightweight, extensible webs (e.g., creped facial tissue paper) are being handled and it has been found necessary to control and/or reduce such supplemental frictional forces to strike a balance between the tension required for control and that which would promote rupture or excessive elongation or uncreping of the delicate web.

In the past, air has been used as a lubricant to reduce the friction between a web of material being processed and turning surfaces used to guide the web along its path of travel. This reduced the work required to feed the web through the web guiding apparatus and thereby effected economies by reducing the power required to propel the web. Air lubrication has the effect of spacing the moving web from a turning surface and thereby reducing the area of contact of the two and thus the friction. This aspect of air lubrication finds particular utility in the field of printing where the printed web would become smeared or otherwise defaced by direct contact of the wet, printed side of the web with the turning surface.

3,366,293 Patented Jan. 30, 1968 Although air has been used to provide lubrication for turning bars, the known techniques and apparatus have not been satisfactory when applied to the handling of porous, stretchable webs such as those employed in the manufacture of facial tissues. The reason for this is that in prior air lubrication devices the air emanates from the turning surface in a radial direction and the web is employed to form one boundary within which the air is confined to form an air cushion between the web and the turning surface. When this approach is used with porous webs, the perviousness of the web permits the radially directed air to pass therethrough with the result that little or no reduction is made in the frictional forces since the air does not form the desired separating layer or cushion. Where the web is stretchable in addition to being porous, the lack of effective lubrication at turning surfaces causes an increase both in web tension and in elongation of the web. If the stretchable web is not elastic or has a low elastic limit, the elongation may be permanent, an effect which could be undesirable.

Accordingly, it is an object of this invention to provide a method and apparatus for regulating the tension in a moving web of material.

It is another object of this invention to provide a preferred air lubrication method and apparatus suitable for use in connection with porous webs and in which the air apertures are adjustable to correct and compensate for varying conditions of the web or air supply.

Briefly stated, in accordance with one aspect of the invention, an air lubricated device is provided for turning a web of material. The turning surface is longer than the width of the web to be turned and has a web-receiving peripheral portion of arcuate configuration. An enclosure disposed adjacent to and on the same side of the web as the bar is in communication with a source of pressurized air and has a longitudinally extending slot therein positioned to direct a stream of air tangential to the bar and toward the Web.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as forming the present invention, it is believed the invention will be better understood from the following description taken in connecton with the accompanying drawing in which:

FIGURE 1 is a fragmentary perspective view of the web tension control apparatus of the present invention showing the parts thereof in operative relationship:

FIGURE 2 is a fragmentary perspective view, partially broken away, of an air-lubricated turning bar forming part of the present invention; and

FIGURE 3 is a cross-sectional view of the air lubricated turning bar taken along the line 33 of FIGURE 2.

Referring now to the drawing and particularly to FIG- URE 1, there is shown the web tension control apparatus 10 of the instant invention. The apparatus is structurally supported by machine frame members 11 or other suitable rigid support means capable of maintaining the 'apparatus in a given position. Suitably attached to members 11, as by bolts or other fastening means, are load cells 12 having resistance strain gages 13 affixed thereto. The load cells are shown to be of the cantilever beam type and so disposed that an external force acting on the beam will set up a bending stress to which the strain gage is responsive. Thus, the force on the load cell may be determined by connecting the strain gage to a suitable circuit (not shown) having a galvanometer calibrated in terms of force units. The manner in which such load cells and strain gages are constructed, as well as the circuitry and manner of operation, are well known in the art and are therefore not described herein. Although described in terms of a bending beam type load cell, the

particular type of force responsive means herein referred to is not critical and may comprise other types of equivalent devices such as, for example, compression-type load cells, tension-type load cells, pieZo-electric crystals, spring scales, and the like, the construction and mode of use of all of which are well known to those skilled in the art.

Attached to and supported by load cells 12 are elongated turning bars 14 and 15 which serve as the turning surfaces for web 16 and provide a means for controlling the friction generated by the passage of the web over the bars and, thereby, the tension in the web. The bars 14 and 15, which have a length at least as great as the width of web 16, may be rigidly attached to load cells 12 as by welding but preferably are mounted thereon in such a way as to permit them to be adjusted and secured in the optimum position relative to the web 16. This can, for example, be accomplished by rotatably mounting the bars and employing setscrews whereby the bars may be secured in a stationary position while in use.

As shown in FIGURES 2 and 3, each of the air-lubricated turning bars 14 and 15 of the instant invention comprise a tube 17 which has a highly polished surface to minimize friction between the bar and the moving web. In FIGURE 2, the drawing has been simplified by omitting end closure plugs which seal the tube 17 interiors at each end of the bars 14 and 15. The bar may be of any convenient size capable of carrying a sufiicient amount of pressurized air used for lubrication purposes and may be of any shape so long as it includes an arcuately configured web-receiving portion over which the web 16 passes. A small diameter tube, while presenting the smallest surface area in contact with a web which wraps partially therearound, may not pass the required airflow for sufficiently reducing the friction between the web and the bar; a large diameter tube which will pass the required quantity of air may result in excessive drag forces on the web due to the greater contact area which results when the same angle of wrap is present. In using such a bar for turning webs of creped tissue paper of the character used to make facial tissues, a stainless steel tube having an outside diameter ranging from about 1" to 4", preferably about 2 /z inches, an inside diameter ranging from about /3" to 3%, preferably about 2 /8 inches, and an exterior surface roughness of less than about 15 microinches and preferably about eight micro-inches or less has been found to be suitable.

A series of aligned radial holes 18 extend through one side of the wall of tube 17. These holes serve to distribute the compressed air which flows from a compressor or other source of pressurized air (not shown) into each tube 17 through a hose 2:: attached to a suitable fitting on an end closure plug (not shown). The other end of each tube 17 is also closed, as indicated above, by another end closure plug so that the air must fiow through holes 18 in order to escape. Holes 18 are preferably countersunk to aid in diffusing the flow and thereby promote mixing of the air from the holes 18 so that a relatively uniform flow field results without local high pressure areas. The size and number of air holes 18 is preferably such that the sum of the cross-sectional areas of the individual holes 18 is less than about one-half the internal cross-sectional area of tube 17 so that the air distribution through the holes 18 is uniform along the length of tube 17. If the areas of holes 18 were too large in relation to the tube area, the flow rate would be unbalanced across the tube since the holes closest to the end at which the air enters tube 17 would pass greater quantities of air than those near the distal end.

Fastened to tube 17 by means of screws 19 or the like is a deflector 29 which is an angularly configured sheet metal member adapted to form a substantially triangular enclosure in combination with the periphery of tube 17. The specific configuration of the deflector is not critical and the term angularly configured is intended herein to mean any shape capable of defining an enclosure when applied to tube 17. The deflector is located immediately adjacent the point at which the web 16 meets the tube 17. An elastic gasket 21 is positioned between deflector 26 and tube 17 to prevent air leakage therebetween and a resilient, imperforate end cap 22 conforming in shape to the cross-section of the enclosure formed by deflector 2G and tube 17 is aflixed to each end of the deflector 20, as by cementing, to prevent axial flow therefrom. Closed cell sponge rubber has been found to be a satisfactory material from which to make end cap 22 because it has the necessary resilience to conform in shape with tube 17 and has the requisite imperviousness to preclude the leakage of air therethrough.

The edge 23 of deflector 20 is not sealed and may be moved from tube 17 to form a slot or nozzle through which the lubricating air flows, being directed between the web 16 and tube 17 on the meeting side thereof to reduce the frictional drag therehetween by means of air lubrication. The preferred location of the edge 23, and therefore of the slot, in relation to the web infeed is described hereinbelow. Deflector 20 is preferably made from a relatively resilient material such as, for example, 16 gauge #304 stainless steel so that when affixed to tube 17 by screws 19 it tends to press edge 23 against the outside surface of the tube and if the edge is forced outwardly a distance of up to about A inch it will return to its original position when the force is removed. With the turning bar configuration herein described, the angle with which the web may wrap the bar can range from barely touching to up to and including about 200 of arc, depending upon the size of the deflector.

It is not essential that the tube 17 form a part of the enclosure and it is Within the scope of the present invention to construct the enclosure in any manner whereby to present an air conducting means having a slot or nozzle through which a stream of air may be directed in accordance with the instant teachings. This can include a separate enclosure or one forming an integral part of the turning bars 14 or 15, Moreover, while the slot is preferably made adjustable, such is not a necessity since the turning bar has utility in and of itself when the size of the slot is fixed and since, for example, adjustment of flow rate can be controlled within limits by varying the pressure of the air supply.

The edge 23 of deflector 20 is spaced from tube 17 a sufiicient distance to permit the passage of the air flow rate required to effect the desired reduction in friction. This spacing can be accomplished and adjusted as necessary by the wedging action of adjusting screws 24 which have one end contacting the tube 17 of bar 14 or 15. Adjusting screws 24 are rotatably carried in and guided by retaining nuts 25 which are spaced along and affixed to deflector 20, as by brazing, and positioned on the part of the triangular enclosure which is opposite the angle formed by edge 23 and the exterior of tube 17. By extending or retracting adjusting screws 24 the relative positions of edge 23 and tube 17 can be varied to adjust the width of the slot and thereby increase or decrease the air flow rate therethrough. The slot spacing range found most suitable for use with creped tissue webs of the type used for making facial tissues and which permits the passage of the requisite air flow rate is from about & to about /s with about V being preferred.

The most important factor affecting the amount of frictional drag which is generated between the web 16 and a turning bar 14 or .15 is the volume of air which is directed therebetween. The volume of air which is required can range from about 1 cubic foot per minute per foot of bar length (c.f.rn./ft.) to about c.f.m./ft. at standard temperature and pressure conditions (70 Fahrenheit and 14.7 pounds per square inch absolute) depending upon the nature of the web material being handled. For creped tissue webs the preferred range of air flow is from about 5 c.f.m./ft. to about 30 c.f.m./ft. As the density and imperviousness of the web to air increases, the quantity of air required diminishes since less air is lost through the web.

The amount of air which is required to provide a film suflicient to reduce the frictional drag between the web and the turning bar is considerably less when the air is introduced tangentially than it is when the air is introduced radially. When a turning bar having radial introduction of air was used with a porous web, the quantity of air so introduced was found insufficient to reduce the friction any appreciable amount since the air tended to flow through the web and was therefore unavailable to provide adequate lubrication. In view of this, it was surprising to find that a large quantity of air was not required when the air is introduced tangentially. To point up the difference in results between the tangential and radial application of lubricating air, experiments were performed with the herein described web tension control device on porous creped tissue paper having a basis weight of about pounds per 3000 square feet. It was found that when there was no air flow the coefficient of friction between the bar and the web was about 0.30, while with an air flow rate of about 27 c.f.m./ft. directed tangentially with respect to the bar and introduced in the general direction of web travel at a point spaced about & inches from the Web, the coefficient of friction between the web and the bar was 0, i.e., the web was completely separated from the bar and was floating on a cushion of air. On the other hand, introducing air radially through a single slot in the bar and positioned in the optimum location for minimum drag, which is at a point about 30 of are beyond the point where the web first touches the bar, the coefficient of friction was reduced from 0.30 to about 0.27 at an air flow rate of about 27 c.f.m./ft. The lowest coefficient of friction which was obtained with this bar when using radial introduction of air was about 0.11 at an air flow rate of about 280 c.f.m./ ft.

A traveling web carries along some of its own lubricating air in the form of a boundary layer. This boundary layer of air travels at or near the speed of the web and varies in thickness according to the web speed and the uninterrupted distance the web has traveled, tending to increase in thickness as distance traveled increases. The thickness of this boundary layer on commercial papermaking machines generally is in the range of from about ,4 inch to about 4 inch in thickness, depending upon the web speed and the uninterrupted distance it has traveled. It has been found that the placement of the edge 23 of deflector 20 (and therefore the slot through which the stream of lubricating air flows from the enclosure) from the web a distance approximately equal to the thickness of the boundary layer, represented by a in FIGURE 3, results in the lowest air flow requirement in conection with the present invention for a particular reduction of friction. If the slot is too close to the web, the air flowing through the slot tends to flush away and dissipate the boundary layer, therefore requiring more air from the slot to compensate for the boundary layer air which was lost. Similarly, if the slot is too far from the web, the air flow therefrom has a greater area in which to diffuse and migrate beyond the ends of the web where it is not effective for lubrication. The amount of boundary layer air available for lubrication purposes is also affected by the velocity of the air issuing from the slot and in order to utilize the maximum amount of such boundary layer air, the preferred velocity range for maximum efliciency is within about :25% of the web speed.

As is shown in FIGURE 3, the slot is disposed so that the air flows tangentially with relation to the outer surface of tube 17 portion of the bar 14 or 15. This placement of the slot makes use of a phenomenon known as the Coanda effect in which a jet of fluid flowing near a surface attaches itself to the surface, provided there is not a severe discontinuity therein such as a 90 turn. Actually, the Coanda effect involves a slight separation of the fluid flow from the surface at some point close to the place where the fiuid jet is introduced. This separation causes a low pressure eddy current which results in a transverse pressure gradient which tends to keep the jet attached to the surface In the present apparatus, the air jet emanating from the slot tends to follow the periphery of the tube 17 portion of bar 14 or 15 for an arc of from about to about '120 with the motion of the web and the contribution of the boundary layer air combining to effect an air cushion for substantially the remainder of the contact arc.

The frictional force between the web and the bar 14 or 15 acts in a direction opposite to the direction of web travel causing a drag on the web which tends to increase the tension therein. For a creped web, since a web of this character is extensible, the increase in tension is manifested by some elongation of the web in the machine direction with an accompanying reduction in the degree of crepe. However, the turning bars of the instant invention are capable of being operated with or without friction, depending upon the amount of air flow used. Thus, by varying the air flow rate, the tension added to the web, and therefore the elongation and change in degree of crepe, can be determined and controlled to provide a sheet having the desired physical properties.

The apparatus shown in FIGURE 1 permits such a determination in that the tension added to the web as a result of the web passing around the turning bars can be readily measured. The load cells adjacent turning bar 14 are force responsive means adapted to indicate a strain which is proportional to the vector sum of the tensile force in the portion of the web located at a point just before the first turning bar and the tensile force in the portion of the web located at a point just after the first turning bar but before it passes over the second bar. Likewise, the load cells adjacent turning bar 15 will indicate a strain which is proportional to the vector sum of the tensile force in the web just before it wraps the second bar and the tensile force in the web just after it leaves the second bar. If there is friction between the web and the bars, the tensile forces in the web approaching and leaving the bar will be unequal by the amount of frictional drag which results. However, if there is no friction these forces will be equal. If the readings of the load cells at the second turning bar are subtracted from the corresponding readings of the load cells at the first turning bar, the result is the tension added to the web by reason of its having passed over the apparatus. This follows since in the algebraic manipulation of the load cell readings the tension of the web in the area between the two turning bars is common to both sets of load cells and therefore in subtracting the readings of the load cells this force cancels itself and what remains is the difference in the tension of the web after it leaves the second turning bar and the tension before it wrapped the first bar, or the tension added. The tension added can then be related to degree of crepe or elongation, as desired.

As stated above, it has been found desirable that extensible webs be handled with some friction on the bars in order to maintain control of the web throughout the system. When using the instant apparatus in a system in which creped, facial tissue paper is being handled, for example, the tension added by the tension control apparatus should be about midway between the point where no tension is added, i.e., completely frictionless opera tion, and the point where the friction added will cause the web to break. The tension is regulated by adjusting the flow rate of the air stream emanating from the deflector 2t} enclosure adjacent the turning bar 14 to thereby change the extent of air lubrication and provide the desired tensile force increase at the turning bars 14 or 15. For example, when handling a creped facial tissue web which would break when a drag force of about 40 grams per inch of web width is imposed thereon, the preferred tension increase value corresponds to a drag force of about 20 grams per inch of width. 1f the air flow rate were increased to the point where there was no friction, the web would be completely spaced from the bar and because of local air flow irregularities along the length of the bar, it would be possible for some web instability, such as fluttering, to develop. This instability may cause web rupture.

The output of the load cells can be read individually and then by subtracting the force values for each turning bar the added tension can be obtained. On the other hand, the load cells can be electrically connected in subtractive relationship, in a manner well known in the art, to give the added tension value directly. The apparatus can also be arranged in a closed loop control system of a type well known to those skilled in the art so that the desired amount of tension to be added is automatically controlled by having the load cell output compared with a set point on a controller which would generate an error signal if the load cell output did not equal the set point value, the error signal being used to actuate a valve which controls the air flow to the turning bars and thereby corrects the tension added so that load cell output coincides with the set point value.

Although the foregoing discussion and description of the method and apparatus forming a part of the instant invention have included references to creped tissue webs, the method and apparatus are also suitable for use with other types of webs such as, for example, kraft paper or plastic films. In addition, it will be recognized that the turning bar structures described also have utility as frictionless guides adapted to change the direction of a moving web and thus may be employed independently of a tension measuring system. It will also be recognized that while the embodiment described employs adjustable air lubrication with both bars, it is feasible to construct the tension measuring and adjusting system using air lubrication on only one bar and/or to limit adjustability of air flow to that in connection with but one bar.

While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention and it is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

What is claimed as new is:

1. A method of regulating the tension in a moving web, said method comp-rising:

(a) passing said web successively around a portion of each of a pair of turning bars;

(b) directing a stream of air between the web and at least one of the turning bars to reduce the frictional drag therebetween by means of air lubrication;

(c) measuring the tensile forces imparted to said web by frictional forces at said turning bars; and

(d) adjusting the flow rate of said stream of air whereby the exten of air lubrication is changed to provide the desired tensile force increase at said turning bars.

2. An air-lubricated device for turning webs of material through angles of up to and including about 200", said device comprising:

(a) an elongated bar at least as long as the width of the web which is to be turned and having a webreceiving portion on the periphery thereof which is of arcuate configuration; and

(b) an enclosure disposed next to and on the same side of the web as said elongated bar, said enclosure being in communication with a source of pressurized air and having a longitudinal slot therein of a length at least about as great as the web width, said slot being positioned adjacent the web as it approaches said web-receiving portion and oriented to direct a stream of air tangentially of said bar toward said web and on the meeting side of said web and said bar.

3. The device of claim 2 wherein said longitudinal slot is spaced from said web a distance corresponding to the thickness of the boundary layer of air carried with said web at a point immediately upstream of said bar.

4. The device of claim 2 wherein said longitudinal slot is spaced from said web a distance of from about A to about inch.

5. The device of claim 2 wherein said elongated bar is hollow and forms a conduit through which said pressurized air flows to said enclosure, and said enclosure comprises a resilient, angularly configured deflector attached to said bar along one longitudinal edge thereof and having end caps mounted thereon, said deflector having a remaining longitudinal edge spaced from said bar to form said slot therebetween.

6. The device of claim 5 wherein said deflector is provided with a wedge member having one end contacting said bar and adapted to change the relative positions of said remaining longitudinal edge of said deflector and said bar whereby the width of said slot is adjusted.

7. Apparatus for controlling the tension in a moving web, said apparatus comprising:

(a) at least two turning bars disposed in spaced relationship to one another and adapted to have a web successively wrap a portion of each bar;

(b) at least one enclosure connected to a source of pressurized air and adapted to direct a moving stream of air between the web and one of said turning bars to effect air lubrication therebetween;

(c) force responsive means attached to said turning bars and adapted to measure the forces acting thereon;

(d) support means for rigidly positioning said force responsive means; and

(e) means for adjusting the rate of flow of air to said enclosure whereby to regulate the frictional drag forces acting on said Web as it proceeds over said bars.

8. The apparatus of claim 7 wherein each of the turning bars is at least as long as the width of said web and has a web-receiving portion on the periphery thereof which is of arcuate configuration and wherein said enclosure is disposed next to and on the same side of the web as said one turning bar and has a longitudinal slot therein of a length at least about as great as the web width, said slot being positioned adjacent the web as it approaches the web-receiving portion of said one turning bar and oriented to direct the flow of air therethrough tangentially of said one turning bar toward said Web.

References Cited UNITED STATES PATENTS 2,258,659 10/1941 Mosler 22677 X 2,945,637 7/1960 Derrick et al. 3,053,425 9/1962 Baines 226-7 X ALLEN N. KNOWLES, Primary Examiner. 

